Recently, the absolute position has been detected for the crank angle and the cam shaft angle of the engine for controlling the automobile engine. An apparatus for detecting the absolute position is constructed such that an annular magnetic encoder is concentrically fixed to a rotary side, the annular magnetic encoder including an equal pitch magnetized region formed by alternately forming the N pole and the S pole at an equal pitch in the circumferential direction and an unequal pitch magnetized portion in which a part of the magnetized region is an unequal pitch portion of the N pole or the S pole and such that a magnetic sensor is provided at a stationary side so as to face the annular magnetic encoder (see the patent document 1). Such an absolute position detection apparatus detects the magnetic change in the equal pitch region and the unequal pitch portion accompanied with rotation of the rotary side to determine the detection output at the unequal pitch magnetized region as an origin position and detects the absolute position of the crank angle and the cam shaft by the relative difference with the pulse number of the detection output at the equal pitch magnetized region.
FIG. 6a and FIG. 6b show a partial development view of one embodiment of the magnetic encoder used for the above-mentioned absolute position detection apparatus and the waveform chart of the magnetic flux density of the magnetic encoder. FIG. 6a is a partial cutaway view of an annular magnetic encoder 50 in which an unequal pitch magnetized region 51 of the S poles equivalent to a plurality of magnetic poles (odd number, for example 5 in the figure) at a part of an equal pitch magnetized region in which the N pole and the S pole are formed alternately at an equal pitch along the circumferential direction. The both sides of the unequal pitch magnetized region 51 are connected with the equal pitch magnetized region 52 in which the N poles and the S poles are alternately formed at an equal pitch. The region comprising the unequal pitch magnetized region 51 and each one N pole at both sides is defined as the unequal pitch region (portion enclosed with the heavy solid line in the figure) and the area ratio of the unequal pitch magnetized region 51 for the unequal pitch region 53 becomes about 71%. FIG. 6b conceptually shows the waveform chart of the magnetic flux density detected by relatively scanning the magnetic sensor in the circumferential direction of the annular magnetic encoder 50 along the sensor measurement line L (a center line in the width direction of the annular magnetic encoder 50) in FIG. 6a.     Patent Document 1: German Patent Publication No. 19800774A1